An optical receiver in which a photodetector serves as a receiver element is one of the key element in an optical fiber transmission network. Optical receivers, in general, function to convert optical signals into electrical signals. A typical optical receiver includes a photodetector connected to the input of an amplifier (e.g., a transimpedance amplifier). The photodetector converts the optical signal it receives into an electric current that is supplied to the amplifier. The amplifier then generates at its output a voltage or current that is proportional to the electric current. The photodetector is typically either an avalanche photodiode (APD) or a PIN (p-intrinsic-n) photodiode. With the recent spread of broadband networks, optical receivers (and optical transmitters) have increased in speed, typically increasing in bit rate from 622 M bps to 2.5 Gbps. More recently still, bit rates up to 10 Gbps are beginning to be widely used.
The sensitivity of optical receiver generally needs to remain constant as the bit rate increases so that the network does not need to be reconfigured for the higher bit rates. For example, optical receiver detectivity for bit rates of 622 Mbps and 1.25 Gbps using a PIN diode is about −32 dBm and −29 dBm, respectively. In addition, optical receiver detectivity for bit rates of 2.5 Gbps is about −26 dBm using a PIN diode and about −32 dBm using an APD. As many systems at 1.25 Gbps are upgraded to 2.5 Gbps, one concern is the weak sensitivity of the optical receiver at 2.5 Gbps. To enhance receiver sensitivity APDs are often preferred because of their superior power sensitivity in comparison to PIN photodiodes. Unfortunately, APDs are generally more costly and less reliable than PIN photodiodes. In addition, APDs can be more difficult to tune and calibrate, in part because its avalanche multiplication factor varies with ambient temperature.
Accordingly, it would be desirable to provide an optical receiver that employs a PIN photodiode but which has an enhanced sensitivity suitable for use in higher speed networks such as those operating at 2.5 and 10 Gbps, for example.